The present invention relates generally to electrical equipment, and in particular relates to systems and methods for cooling.
Maintaining electrical devices and equipment within specified temperatures is often an important requirement for maintaining the operability of those devices. In the field of microelectronics, for example, microchips such as microprocessors must be maintained at or below maximum temperatures during operation to prevent the self-destruction of the microchips. Accompanying recent and continuing increases in the processing power and operating speeds of microchips led to increases in the power dissipation of those microchips. For example, new high-powered chips can dissipate at least 10 Watts of power per chip and have heat fluxes as high as 200 W/cm2. Consequently, the cooling of microchips during their operation is becoming even more critical as the performance of those microchips becomes even greater.
Numerous techniques for cooling currently exist. With respect to the cooling of microchips in particular, many existing cooling techniques incorporate large and costly heat spreaders and heat sinks, which impose limits on package size and functionality. Other techniques involve directing one or more jets or streams of cool air or other gas (or liquid) at or along the one or more microchips that require cooling. Referring to FIGS. 1-4 (Prior Art), one conventional solution for microelectronics cooling is a channel-flow system, in which a cool air jet 10 is directed between two circuit boards 12, 14 across the one or more microchip devices 16 that require cooling. The cool air jet 10 is generated by way of a conventional blower, fan or other standard pumping device 11. Although simple to implement, the channel-flow system is limited in that it requires large pumping powers to provide effective cooling (e.g., Ppump=0.0348 kg/s).
Further, as air flow passes along the circuit board 12, heat energy removed from upstream microchip devices raises coolant temperature such that downstream devices are not exposed to as low of a coolant temperature. As a result, downstream devices do not receive appropriate cooling, and they can rapidly reach prohibitive temperatures that may lead to failure and/or damage. As shown in FIG. 2, which shows increasing heat intensities as increasingly light shades of gray, the temperature of the air coolant is increased as it passes by and absorbs heat from device 16a, for example. Consequently, the cooling experienced by any downstream devices such as devices 16b-16d is considerably lessened. Further, because the air flowing along each device 16 (such as device 16a) picks up heat energy as it passes along the respective device, the heat transfer coefficient experienced by each respective device along its top face decreases significantly along the surface of the device as one progresses in the direction of air flow (see FIG. 3). Consequently, the time-averaged temperature along the surface of the device increases significantly as one moves along the device in the direction of air flow (see FIG. 4). Additionally, in cases where not all of the microchip devices 16 require the same amount of cooling, the channel-flow system wastes air and pumping power on devices that do not require cooling. Consequently, the channel-flow system has a limited efficiency as a system for cooling microchips.
Turning to FIGS. 5-8 (Prior Art), a second conventional system for microelectronics cooling involves directing a steady impinging jet of air 20 (generated by way of pumping device 11 as shown in FIG. 5) from a rectangular slot 18 in a top circuit board 24 down toward a bottom circuit board 22 on which one or more microchip devices 26 are situated. Typically, a jet is characterized as a unidirectional fluid flow, that is unconstrained so that the width or diameter of the fluid flow increases along the direction of flow. A jet has a central tip at the middle of a cross-section of the jet at a point where that middle point of that cross-section intersects an object in the path of the jet. A central core of the jet is a central portion of the jet along the direction of flow where the laminar flow occurs. At a central tip of the jet 20, which impinges the surface of the single microchip device 26 that is supported by the bottom circuit board 22, is a stagnation point 27. This is the point of the jet at which the greatest pressure of coolant is applied, and is characterized by the highest heat transfer coefficient (thus highest heat removal capacity). The steady impinging jet system is a better cooling solution than the channel-flow system, because similar time-averaged heat transfer coefficients and temperatures can be obtained along the top surfaces of the microchip devices 26 with a pumping power that is about 6.5 times lower than for the channel-flow system (e.g., Ppump=0.00548 kg/s vs. 0.0348 kg/s), and with lower velocity air flows (compare FIGS. 7 and 8, respectively, with FIGS. 3 and 4). Further, the steady impinging jet system is better than the channel-flow system also because the cooling of the microchip devices 26 occurs in a more uniform manner along the top faces of the microchip devices (see FIGS. 7 and 8).
Nevertheless, the steady impinging jet system also is limited in its effectiveness for cooling microchips. As shown in FIG. 6, the heat removed from the microchip device 26 undergoing cooling is confined to a region above the bottom circuit board. This region is identified as the thermal boundary layer developing along the top surface of the device and board, that is, where the distance between the top of the device/board and the dark region above it is thin. Although cooling occurs efficiently where this boundary layer is thin, efficient cooling does not occur where this boundary layer becomes thicker, as it does on either side of the microchip device 26. Consequently the temperatures along the circuit board 22 on either side of the central microchip device 26 are not reduced as much by the operation of jet 20, and other microchip devices placed downstream on either side of the central microchip device may as a result be exposed to excessive temperatures. This is the case, for example, at points 21 in FIG. 6.
Referring to FIGS. 9-12, a third conventional system for cooling microchip devices 36 involves directing two steady impinging jets 30a,30b parallel to one another from respective rectangular slots 28a,28b towards respective microchip devices 36a,36b. Each of the rectangular slots 28a,28b has a width of 1 centimeter (half that of rectangular slot 18) and each jet 30a,30b has a pumping power that is approximately half that of jet 20, so that the combined effect of the steady impinging jets is similar to that of jet 20 with respect to FIGS. 5-8. Using multiple jets 30 is advantageous relative to the single steady impinging jet system of FIGS. 5-8 because the heat transfer coefficient is maintained at a more uniform level over a wider region, over two (or more) devices 36 (see FIG. 11). Likewise, the temperature along the top surface of the microchip devices 36 and the circuit board 32 supporting those devices is reduced over a larger area as shown by FIGS. 10 and 12.
However, the multiple jet system is less efficient than the single steady impinging jet system in terms of the amount of cooling that occurs given the same amount of total air flow and power for generating air flow. The multiple jet system additionally suffers from possible increases in temperature, especially at locations in between the individual jets due to the interaction of the jets. Because of the interaction between the jets, there is a slight shift in the flow, as well as to a recirculation region (with hot air) between the two devices, which reduces the heat transfer coefficient and hence provides poor cooling at device edges. Further, the multiple jet system suffers from the same problem as the single jet system insofar as efficient cooling only occurs where there is a thin thermal boundary layer. Since heat is confined to a thin thermal layer, downstream components would be negatively impacted, similar to the examples of the second system previously discussed.
While each of the three conventional systems for cooling microchips shown in FIGS. 1-12 have relative advantages and disadvantages, the systems require large pumping powers to generate the air flow required to cool the microchip devices. Further, each of the systems has one or more limitations in terms of the uniformity of the cooling that occurs along the region that is desired to be cooled. Consequently, there remains a need for a system and method for more effectively cooling microchips as well as other devices generally. In particular, there remains a need for a system and method for cooling which is more efficient at cooling than conventional systems and at the same time provides a more uniform cooling effect than conventional systems.